The present invention relates to a method for manufacturing SOI wafer and thus-manufactured SOI wafer.
There has been a general trend of handling high-frequency signal of several hundred MHz or above in recent mobile communication typically using cellular telephones, which strongly demands semiconductor devices with excellent high-frequency characteristics. Semiconductor devices such as CMOS-IC and high-voltage IC typically employ so-called SOI wafer comprising a silicon single crystal substrate (also referred to as xe2x80x9cbase waferxe2x80x9d hereinafter), a silicon oxide layer (buried oxide film) formed thereon, and another silicon single crystal layer stacked further thereon as an SOI (silicon-on-insulator) layer. For the purpose of fabricating semiconductor devices for high-frequency use on the SOI wafer, it is necessary for the base wafer to be composed of a high-resistivity silicon single crystal in order to reduce high frequency loss.
One representative process for manufacturing the SOI wafer relates to bonding process. According to the bonding process, a first silicon single crystal substrate (also referred to as xe2x80x9cbond waferxe2x80x9d hereinafter), which provides an SOI layer affording device formation area, and a second silicon single crystal substrate which serves as a base wafer are bonded so as to locate a silicon oxide film in between, and the bond wafer is then reduced in the thickness thereof so as to be thinned to a film having a predetermined thickness, to thereby convert the bond wafer to the SOI layer.
In the above-described bonding process, a bonding interface between the base wafer and the bond wafer may sometimes catch foreign matters such as particles. Such foreign matters accidentally residing on the bonding interface may induce lattice defect such as void, degraded wafer characteristics typically due to diffusion of impurities, and degraded bonding strength between both substrates. The substrates are thus bonded in a clean room (or in a clean area) so as to avoid the contamination of foreign matters into the bonding interface. In the manufacture of SoI wafer by the bonding process, it is a general practice to form the silicon oxide film only on the surface of the bond wafer, and then bond the base wafer with the bond wafer so as to locate the silicon oxide film in between.
Another known problem resides in that the clean room, which is a site of the wafer bonding, usually contains in the atmosphere thereof boron which is derived from the air filter, and which boron can be incorporated as an impurity into the bonding interface. Boron thus incorporated into the bonding interface diffuses during high-temperature annealing (bonding annealing) for raising bonding strength or during annealing for forming devices. In this point of view, the foregoing bonding process in which the silicon oxide film is formed only on the bond wafer hardly affects the devices since the boron diffusion into the SOI layer (device forming area) is blocked by the silicon oxide film. This is one reason why the foregoing bonding process in which the bond wafer, only on which the silicon oxide film is formed, is bonded with the base wafer is widely accepted. Whereas the bonding interface between the base wafer and silicon oxide film still suffers from adsorption of boron derived from the air filter, so that the boron diffusion into the base wafer is still inevitable during the foregoing bond-annealing.
The above-described boron diffusion into the base wafer has not attracted much attention so far as a silicon single crystal substrate having a normal-to-low resistivity is used as the base wafer. The problem of degradation of high-frequency characteristics however arises in the SOI wafer for high-frequency use, since the base wafer has a resistivity of as high as hundreds to thousands xcexa9xc2x7cm, and the resistivity of an interfacial portion of the base wafer several micrometers deep from the interface with the silicon oxide film may considerably be lowered due to the boron diffusion.
One solution for the foregoing problem is disclosed in Unexamined Japanese Patent Publication No. 2000-100676, in which SOI wafer is manufactured by properly selecting types of the air filter used for introducing air into a clean room to thereby control the amounts of boron as a p-type impurity together with n-type impurity in the bonding atmosphere. The methods disclosed in the patent are such that:
1. using a boron-free filter system which comprises a PTFE filter and a boron-adsorptive chemical filter irrespective of conductivity type of the base wafer. Using the boron-free filter is beneficial to suppress boron-induced degradation in resistivity of the base wafer particularly for the case that the base wafer comprises a p-type silicon single crystal substrate having a high resistivity; and
2. using a boron-releasable HEPA filter when the base wafer comprises an n-type silicon single crystal substrate having a high resistivity. Degradation of the resistivity is avoidable even if boron is adsorbed since the adsorbed boron is compensated by the n-type dopant contained in the n-type silicon single crystal substrate.
The foregoing method 1 is however disadvantageous in that the boron-free filter system is expensive and is less economical. While the method 2 is applicable to the case the n-type base wafer is used, it is of course inapplicable to the case the p-type base wafer is used. The paragraph 0150 of the foregoing patent publication also describes difficulty in use of the HEPA filter for the high-resistivity, p-type wafer. It is also anticipated that even the resistivity of the n-type wafer may degrade unless concentrations of the n-type dopant and the filter-derived adsorbed boron are properly balanced.
An object of the present invention therefore resides in providing a method for manufacturing SOI wafer less causative of degradation of resistivity of the base wafer even when a high-resistivity silicon single crystal substrate of either conductivity type is used as the base wafer and is bonded in a boron-containing atmosphere; and also in providing an SOI wafer producible by such method, capable of retaining high resistivity of the base wafer by localizing boron incorporated during the bonding, capable of retaining desirable electrical characteristics of the SOI layer, and excellent in high-frequency characteristics.
To solve the foregoing problem, the method for manufacturing SOI wafer of the present invention comprises a bonding step including a process of bringing the main surfaces of a first silicon single crystal substrate and a second silicon single crystal substrate, each of such main surfaces having previously formed thereon a silicon oxide film, into close contact so as to locate such silicon oxide films in between; and a thickness reducing step for reducing the thickness of such first silicon single crystal substrate to thereby convert it into an SOI layer, wherein such second silicon single crystal substrate comprises a silicon single crystal substrate having a bulk resistivity of 100 xcexa9xc2x7cm or above, and such process of bringing the main surfaces into close contact in such bonding step is proceeded in an atmosphere of a clean air supplied through a boron-releasable filter.
The present invention employs a silicon single crystal substrate having a bulk resistivity of 100 xcexa9xc2x7cm or above as the second silicon single crystal substrate (corresponded to the base wafer), and dare employs, in order to bring such substrate into close contact, an atmosphere containing a high concentration of boron derived from the air filter, which is usually found in ordinary clean rooms. The atmosphere is composed of a clean air supplied through a boron-releasable filter (which is exemplified by HEPA filters disclosed in Unexamined Japanese Patent Publications Nos. 10-165730 and 8-24551). In the present invention, the silicon oxide film is respectively formed on both of the second silicon single crystal substrate and the first silicon single crystal substrate (corresponded to the bond wafer), and both silicon oxide films are then brought into contact with each other.
The SOI wafer of the present invention comprises a silicon single crystal substrate; a silicon oxide film formed on the main surface of such silicon single crystal substrate; and an SO layer comprising a silicon single crystal layer formed on such silicon oxide film, wherein such silicon single crystal substrate has a bulk resistivity of 100 xcexa9xc2x7cm or above, and such silicon oxide film has a depth profile of boron concentration in which the boron concentration reaches maximum at a thickness-wise position more closer to the silicon single crystal substrate away from the center of the film thickness.
According to the method for manufacturing SOI wafer of the present invention, the bonding interface is formed within the silicon oxide film, which means that boron which resides in the bonding atmosphere is confined within the silicon oxide film (buried oxide film). Since the diffusion coefficient of boron in the silicon oxide film is small, the boron diffusion into the SOI layer and silicon single crystal substrate (base wafer) can successfully be suppressed even after high-temperature annealing for raising bonding strength of the oxide films.
It is preferable herein that the thickness of the silicon oxide film formed on the base wafer is smaller than the oxide film formed on the bond wafer. By manufacturing the bonded SOI wafer based on such definition of the thickness of the oxide films on both wafers, the bonding interface is formed at a thickness-wise position more closer to the base wafer away from the center of the film thickness. This ensures the SOI wafer to have more stable device characteristics. The next paragraphs will describe the reason why.
To prevent the high-frequency characteristics of the SOI wafer from being degraded, it is necessary to avoid lowering of resistivity of the base wafer as described in the above. The present invention thus provides an effective measure whereby the oxide films are mutually bonded so as to confine the atmospheric boron into such oxide films. There is, however, still an apprehension that boron confined in the bonding interface may diffuse in the oxide film to reach the SOI layer or base wafer depending on various conditions such as boron concentration in the bonding atmosphere, bonding annealing, annealing for device fabrication, and thickness of buried oxide film necessary for device characteristics.
The concentration of boron possibly diffused through the oxide film might be fairly small as compared with the boron concentration in the vicinity of the bonding interface as described in the above, but even such small amount of boron can adversely affect the device characteristics ensured by the SOI layer if diffuses thereto, since the absolute amount of dopant intrinsically contained in the SOI layer as thin as 1 xcexcm or less is quite small. Moreover, when there is a need for the thickness of buried oxide layer of as thin as 0.1 xcexcm or less, thinner buried oxide film makes the bonding interface closer to the SOI layer. In a microscopic view, the bonding interface, however, has sites of incomplete chemical bond, and fixed charge ascribable to such sites may adversely affect the SOI layer in which device formation area will be reserved. Considering the above, the bonding interface is preferably formed in the buried oxide film closer to the base wafer.
On the other hand, the base wafer will suffer from only a slight degree of lowering in the bulk resistivity thereof and will cause only a fairly limited degradation of the high-frequency characteristics if a slight amount of boron diffused through the oxide film may be incorporated therein, since the absolute value of the dopant concentration of such base wafer is relatively large, despite its high resistivity, if a large thickness thereof is taken into consideration. It is also noteworthy that the fixed charge within the buried oxide film will never affect the base wafer which does not serve as active layer of devices.
As judged from the above, the thickness of the oxide film formed on the bond wafer is preferably 0.1 xcexcor more in consideration of effects on the SOI layer exerted by boron diffused through the oxide film, or by fixed charge which resides in the bonding interface. The thickness of the oxide film exceeding 2 xcexcm is, however, not practical since formation of such thick film needs a considerably long annealing time in a normal-pressure thermal oxidation furnace which is widely accepted.
The SOI wafer of the present invention will therefore be such that effectively suppressing the boron-induced lowering of the bulk resistivity of the silicon single crystal substrate, and being preferably applicable to high-frequency devices. It is also economical since use of an expensive facility such as boron-free filter system is no more necessary.
As the second silicon single crystal substrate (referred to as a silicon single crystal substrate in the bonded SOI wafer) employed in the method of the present invention, it is preferable to use a substrate having a resistivity of 100 xcexa9xc2x7cm or above, more preferably 500 xcexa9xc2x7cm or above, and still more preferably 1,000 xcexa9xc2x7cm or above in view of ensuring desirable high-frequency characteristics.
The bonding step in the method of the present invention may include an annealing process which is carried out within a temperature range from 1,150 to 1,300xc2x0 C. so as to achieve sufficient bonding strength. For the purpose of bonding the oxide films with each other, annealing at a temperature below 1,150xc2x0 C. may sometimes result in insufficient bonding strength. More specifically, the SOI wafer obtained after annealing below 1,150xc2x0 C. may show only an insufficient chemical bonding strength when measured by immersing such wafer into an aqueous solution of hydrofluoric acid so as to assess the corrosion status, even after a sufficient mechanical bonding strength is observed typically in tensile strength test. On the other hand, annealing at 1,150xc2x0 C. or above, preferably at 1,200xc2x0 C. or above, ensures a satisfactory level of bonding strength not only mechanically but also chemically. The annealing temperature exceeding 1,300xc2x0 C., however, will be more likely to generate slip dislocation, which is inappropriate since problems in durability of an annealing furnace and metal contamination tend to arise. The annealing temperature is thus preferably set at 1,250xc2x0 C. or below from a practical viewpoint.